The present invention relates generally to the generation of coherent, short-wavelength ultraviolet radiation, and more particularly to a method for generating continuously-tunable UV radiation in the wavelength range 2000-2500 A.
In studies involving photon interaction processes, such as photoionization and photodissociation processes, there has evolved a need for high intensity sources of short-wavelength ultraviolet radiation. There is a particular need for high-intensity, wavelength-tunable sources in the spectral region below about 2500 A, and more especially below about 2300 A.
The development of wavelength-tunable dye lasers and parametric generators spanning the visible and near-infrared regions of the spectrum has made it possible to obtain continuously-tunable, coherent UV in the range from about 4000 A down to about 2300 A by direct frequency doubling in nonlinear crystals. Crystals of ammonium dihydrogen phosphate (ADP) are commonly used for such phase matched second harmonic generation because of the material's high conversion efficiency high transparency and resistance to optically induced damage. However, the shortest wavelength at which phase matching for second harmonic generation can be achieved with ADP is about 2442 A. Shorter wavelengths, down to about 2300 A, can been generated by direct frequency doubling in lithium formate monohydrate (LFM) crystals. LFM has a lower conversion efficiency than ADP, however, an efficiency of about 2 percent at fundamental powers in excess of 50 kW having been reported. Nonlinear crystals with sufficient transparency, damage resistance, and birefringence to allow direct second harmonic generation at wavelengths shorter than about 2300 A are not presently available.
Another approach to the generation of tunable, short-wavelength UV involves summing the fundamental and second harmonic of a fixed wavelength near-infrared laser with the output of a tunable, visible dye laser. Such third order, i.e., threephoton, interaction processes are very inefficient and yield unacceptably low outputs unless very high peak power primary lasers are used to irradiate the nonlinear mixing crystal.